1. Field of the Invention
The present invention is related to a dual tip atomic force miscroscopy (AFM) probe with two mutually isolated conductive tips in close vicinity, for example for use in local resistivity measurements or for determining dopant concentration of a semiconductor. The invention is equally related to a method for producing such a probe.
2. Description of the Related Technology
Resistivity measurements can be performed using one probe (with a back contact) to four conductive probes. In the four-point probe case, current is supplied through the outer two probes while a voltmeter measures the voltage across the inner two to determine the sample's resistivity. While this method is very accurate, its spacial resolution is typically poor and it is normally used to measure the resistivity of uniform materials and thin films.
Resistivity measurements can also be performed between two probes at the expense of accuracy since the measured resistance then includes the contact resistances between the probes and the sample in addition to the sample's resistance. Depth dopant profiles are often measured using such a two-probe technique then referred to as a spreading resistance probe (SRP) technique, described in “Spreading resistance: A quantitative tool for process control and development”, M. Pawlik, J. Vac. Sci. Technol. B 10, 388 (1992). In this case, two probes are displaced in parallel on the sample along the concentration gradient while the resistance between the two is measured. The carrier concentration is then extracted from the resistance. In order to improve the depth resolution, the probes are displaced on a small angle bevel rather than on a cross-section. The resolution and accuracy of the SRP measurement is closely related to the distance between the two probes: the smaller this distance and the smaller the size of the tip's contact zone, the higher are the accuracy and spatial resolution.
In a classic SRP tool, the distance between the two probes is at least 10 μm. The total size of the contact is in the order of 1 μm. The force applied on the probes is about 20 mN. The probes are made out of metal (i.e. OsW). To improve accuracy of the measurement, multiple tip AFM probes have been proposed which allow an SRP measurement with high resolution, in an AFM tool. Document EP899538 for example, discloses an AFM probe with two tips on one cantilever, wherein the tips are created by making two adjacent molds. This complex solution does not allow however to reduce the distance between the tips to the order of 100 nm, as required by current accuracy and resolution requirements.
In the document ‘Air-Bearing Sliders and Plane-Plane-Concave Tips for Atomic Force Microscope Cantilevers’, R. P. Ried et al., Journal of Microelectromechanical systems, vol. 9, no. 1, March 2000, a method is disclosed wherein a single tip is produced by forming and filling a mold in a Si-substrate. The mold is formed by producing a trench in a Si-substrate, filling the trench, patterning an opening in the trench-filling material to one side of the trench, and isotropically etching the Si at the opening. However, the mask alignment for creating the opening needs to be extremely accurate, in order to be able to control the height of the resulting tip.
It is desirable to provide a dual tip AFM probe with electrically insulated tips, wherein the tips are robust and at a smaller distance from each other than in prior art devices, and methods for producing such an AFM probe.